In recent years, with increase in a communication speed and a communication data amount in optical communications, there is a demand for optical modules used in optical communications to achieve a high-speed operation and a high-density structure. As speeding-up and densification are progressed, power consumption of an optical module has been increased and an amount of heat generated in a limited narrow space in the optical module has been increased. That is, an amount of electric power consumed by an optical module has been remarkably increased, which results in a remarkable increase in an amount of heat generated by electronic parts of the optical module. Because an arrangement of internal components must be optimized due to miniaturization of optical modules, it has become difficult to acquire a sufficient space needed for radiation of heat. Particularly, characteristics of a high-speed IC used for controlling and driving an optical module tend to be influenced by an ambient temperature. It is needed for such a high-speed IC to take measures for heat-release so as to maintain the characteristics stable.
A high-speed IC may be provided with a GND electrode (bottom electrode) on a bottom surface of an IC package in order to promote releasing heat generated by an IC chip incorporated therein and to improve a grounding property. An electronic part provided with a GND electrode (bottom electrode) on a bottom surface thereof is referred to as a bottom electrode part. The bottom electrode component releases heat generated by an IC chip incorporated therein through the bottom electrode. Basically, the bottom electrode is soldered to a printed circuit board and a GND pattern, which is capable of releasing a sufficient amount of heat, is provided to the printed circuit board to release heat through the bottom electrode and the GND pattern. Additionally, there is suggested a method of causing a package (a surface opposite to a bottom electrode) of a bottom electrode component to contact with a mechanical part containing a surrounding housing via a silicon heat-release sheet.
Thus, there is an increasing demand for a structure, which is capable of performing an efficient heat-release with respect to the bottom electrode component.
On the other hand, due to an influence of the recent speeding-up of an operation of an optical module, a demand for improvement in the electromagnetic environmental performance (EMI-ESD) has become severe. In order to clear the EMI performance specified by severe standard such as FCC PART15 or CISPR22, a printed circuit board is incorporated into a mechanical component (mainly a housing) of an optical module as much as possible to eliminate an empty space in the housing. Additionally, in order to improve an ESD resistance, it is usual to separate a housing from a grounding part of a printed circuit board.
In a case of an optical transceiver module as an example of an optical module, an excellent EMI characteristic can be achieved by incorporating a printed circuit board into a housing. In such a case, in view of an ESD characteristic, it is necessary to separate the housing from a grounding part of the printed circuit board. Accordingly, the bottom electrode of a bottom electrode component is not permitted to contact with the housing. However, if the printed circuit board is incorporated into the housing, heat-release can be done more efficiently by causing the bottom electrode component to contact with the housing than releasing heat in an interior of the optical module through the printed circuit board.
Thus, it is suggested to divide a housing into a frame grounding part (FG) and a signal grounding part (SG), and causing the signal grounding part (SG) to contact with a bottom electrode of a bottom electrode component. However, a portion between the frame grounding part (FG) and the signal grounding part (SG) of the housing may deteriorate the EMI characteristic. If an insulating plate is provided between the frame grounding part (FG) and the signal grounding part (SG) of the housing, there may be a problem in a high-speed operation of the bottom electrode component because there is no grounding connection to the insulating plate.
Moreover, there is suggested a method of causing a package of a bottom electrode component (a surface opposite to a bottom electrode) to contact with a mechanical component including a surrounding housing via a silicon heat-release sheet. According to this method, separation of grounding and good EMI characteristic can be acquired. However, the heat-release performance may not be improved, even if the mechanical component is brought into contact with the housing via the silicon heat-release sheet, due to a material having an extremely large heat resistance, which is larger than 10° C./W, may be used for the package of the bottom electrode component because the bottom electrode component is based on releasing heat from the bottom electrode. Further, if the housing is brought into contact with the package of the bottom electrode component, static electricity accumulated in the housing may be discharged to the bottom electrode component, which causes a problem of deterioration of the ESD characteristic.
Then, the following methods are suggested as a method of promoting heat-release from the bottom electrode of the bottom electrode component.
It is suggested to release heat from a bottom electrode of a bottom electrode component to a housing through a thin portion of a printed circuit board by reducing a thickness of the printed circuit board in an area where the bottom electrode component is mounted and causing the housing to contact with the thin portion of the printed circuit board from the opposite side of the bottom electrode of the bottom electrode component (for example, refer to Japanese Laid-Open Patent Application No. 2004-140171).
Moreover, it is suggested to release heat of a heat-generating component through a flexible board by mounting the heat-generating part to the flexible bard, which is thinner than a rigid board (for example, refer to Japanese Laid-Open Patent Application No. 2007-294619). According to this method, heat transferred from the heat-generating component is transmitted to a pattern having a large area on the flexible board to release the heat from the pattern.
According to the method of releasing heat by thinning an area of a printed circuit board where a bottom electrode component is mounted, a material forming the printed circuit board is merely thinned but the material of the printed circuit board itself is not changed. The material of the printed circuit board has an extremely low thermal conductivity such as a glass-epoxy in many cases. Thus, a large improvement cannot be acquired even if the printed circuit board is thinned. Additionally, because a process of thinning the printed circuit board by machining only a portion of the printed circuit board is added to the manufacturing process, a manufacturing cost is increased. Further, the thinned portion of the printed circuit board is easily broken, which raises a problem in that it is difficult to maintain good mounting reliability.
According to the method of releasing heat of a heat-generating part to a surrounding area through the flexible board by mounting the heat-generating part onto the flexible board, which is a method of diffusing heat by using a conductive layer of the flexible board, the heat-releasing efficiency cannot be improved if the area of the flexible board is large (that is, the area of the conductive layer is large). Additionally, because the flexible board is deformed easily, there is a problem in that it is difficult to maintain the mounting reliability of the heat-generating component mounted on the flexible board.
Here, there exists a restriction on mounting also in a high-speed IC and an optical input/output device connected to the high-speed IC. It is desirable to arrange the high-speed IC near the optical input/output device on a circuit board. This is because if the high-speed IC is remote from the optical input/output device, a possibility of deterioration of radio frequency characteristic and intrusion of noise is raised. Normally, an optical input/output device is connected to a high-speed IC through a flexible board. However, in many cases, a terminal portion of the flexible board is an area where bending is prohibited. In the bending prohibited area, terminal parts are connected to a rigid board by soldering or connectors, and the entire area of the bending prohibited area is supported by the rigid board by being closely contacted with the rigid board. Thereby, the bending prohibiting area is maintained in a flat state, and, thus, the flexible board is prevented from being bent in the bending prohibiting area.
On the other hand, it is desirable to arrange the bottom electrode component such as a high-speed IC at a position as electrically close to an optical component as possible in order to improve communication characteristics. If possible, it is desirable to arrange the bottom electrode component on a flexible board, which connects an optical input/output device to a rigid board. However, the flexible board may be easily deformed or damaged by an external force. Thus, if the bottom electrode part is mounted on a flexible board, the mounting reliability of the bottom electrode component may be deteriorated due to deformation of the flexible board. Particularly, because the bending prohibited area of the flexible board is weak to an external force, it is required to provide a reinforcing function with respect to an external force.